The present invention relates to a system for controlling the transmission ratio of an infinitely variable transmission for a vehicle, and more particularly to a system for downshifting the transmission.
European Patent Publication No. 0011342 discloses a control system for an infinitely variable transmission. The transmission comprises an endless belt running over a primary and a secondary pulley, each pulley being so designed that the running diameter of the driving belt on the pulley varies in dependency on driving conditions. In the system, a transmission ratio control valve is provided so as to change the transmission ratio in dependency on driving conditions of a vehicle. The control valve comprises a spool biased in one direction by a spring which is compressed by a shift cam rotated in dependency on the depression of an accelerator pedal and biased in the opposite direction by Pitot pressure dependent on the speed of the primary pulley, that is engine speed. The spring biases the spool in the direction which results in downshifting and the Pitot pressure urges the spool in the upshift direction. Thus, during starting of the vehicle, when an accelerator pedal is depressed, the spool is shifted in the downshift direction to provide the acceleration of the vehicle. If the accelerator pedal is greatly pushed down (kickdown) during the driving of the vehicle, the transmission is shifted down to effect an acceleration characteristic. The acceleration characteristic depends on the magnitude of depression of the accelerator pedal and on the shape of the shift cam. It is difficult to make a profile of a shift cam which provides a sufficient acceleration characteristic at every speed of the vehicle, and hence there is a speed range in which the vehicle can not be desirably accelerated.
On the other hand, an infinitely variable transmission connected with an engine through an electromagnetic clutch has been proposed. In such a system, the electromagnetic clutch disengages when the vehicle speed decreases below a predetermined low speed. If the engine is accelerated during such low speed coasting in order to re-drive the vehicle, the engine speed rapidly increases to a high speed because of very light load due to disengagement of the clutch. When the engine speed exceeds a predetermined speed, the clutch engages. At that time the drive shaft of the clutch is rotated at a high speed by the engine, and on the contrary the driven shaft (that is the primary pulley) is rotated at a low speed by wheels of the vehicle through a power transmitting system. Accordingly, when the clutch engages under a condition of a great difference between the speeds of drive and driven shafts, the clutch produces shock or greatly slips, which decreases the acceleration characteristic.